Less known than calcite and silica, sulphide deposits can cause serious efficiency losses in some geothermal fields, especially in the heat exchangers of binary plants. Correct diagnosis of these scales is a prerequisite for an effective programme.
Stibnite (antimony sulphide)
Some reservoir fluids contain trace antimony and dissolved H₂S. Soluble at high temperature, antimony sulphide loses solubility as the fluid cools and forms dark, thin layers with poor heat conductivity, especially at the cold end of exchangers. Even small amounts can cause a noticeable loss of efficiency.
Iron sulphide
Iron released by corrosion reacts with H₂S in the fluid to form black iron sulphide deposits. These deposits add to scale and also indicate ongoing corrosion, so the solution includes strengthening the corrosion programme.
Diagnosis and control
- Scale analysis: determining composition by XRD/XRF or chemical analysis
- Fluid analysis: tracking antimony, arsenic, iron and H₂S levels
- Temperature management: assessing cold-end temperature against precipitation risk
- Chemical programme: composition-matched dispersant and inhibitor applied together with corrosion control
| Scale type | Typical appearance | Likely source |
|---|---|---|
| Stibnite | Dark red to black, thin layer | Antimony and H₂S in the fluid |
| Iron sulphide | Black, powdery or layered | Corrosion + H₂S |
| Mixed scale | Layered structure | Silica, sulphides and corrosion products together |
Frequently asked questions
How can I tell a sulphide scale apart?
Colour and texture give clues, but a laboratory analysis is needed for certainty; you can send us a sample.
Does a silica programme also prevent sulphide scale?
Not always; sulphide scales form by a different mechanism. The programme should be designed around the scale composition.
Is it related to corrosion?
Iron sulphide is directly related to corrosion; controlling corrosion also reduces this scale.
Related guides
- Geothermal power plant chemicals (overview)
- Calcite scaling in geothermal wells
- Silica and metal silicate scaling
- Corrosion in geothermal systems
- Downhole dosing via capillary tubing
- Heat exchanger and ORC fouling
- Scaling in reinjection lines and wells
- Geothermal heating and greenhouse systems
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